This invention relates to Ti-Ni alloys having a shape memory effect, and in particular, to Ti-Ni alloy articles having a reversible shape memory effect and to a method for making the articles.
It is known in the prior art that Ti-Ni alloy has a unique property which is referred to as, so called, "shape memory effect" (S.M.E.). That is, when the alloy having a certain shape is deformed at an appropriate temperature and then heated to a sufficient high temperature, the alloy rapidly recovers the original shape (see U.S. Pat. No. 3,174,851 by Buehler et al).
Since the S.M.E. of Ti-Ni alloy was published by Buehler et al in Journal of Applied Physics, 34 (1963), 1467, many peoples have researched the mechanism of the S.M.E. in Ti-Ni alloy. However, it has not been yet sufficiently elucidated, although the S.M.E. is generally thought to be based on the martensitic transformation of the alloy.
It is known that the Ti-Ni alloy can have additions such as Fe, Cu, or others.
The S.M.E. of Ti-Ni alloy is expected to be applied onto various fields and has been practically used in several applications, but it is disadvantageously one-way in its shape recovery, that is, the original shape at a temperature higher than the temperature (As) for reverse transformation of the martensitic transformation of the alloy is recovered by heating from a lower temperature than As but the shape at the lower temperature is not recovered by cooling from the higher temperature.
If it is possible to provide a two-way shape memory effect or a reversible shape memory effect (R.S.M.E.) to Ti-Ni alloy wherein the original shape at a higher temperature is recovered by heating from a lower temperature while another shape at a lower temperature is also recovered by cooling from the higher temperature, application fields of Ti-Ni alloy may be extended.
As disclosed in "On the Mechanism of Reversible Shape Memory Effect in Cu-Zn-Al Alloys" by Takezawa and Sato, PROCEEDING OF THE INTERNATIONAL CONFERENCE ON MARTENSITIC TRANSFORMATIONS 1979, p.p. 655-660, in studies on shape memory alloys such as Ti-Ni, Cu-Zn-Al-Ti, Cu-Zn-Ga, Ni-Al, Cu-Zn and Cu-Zn-Al alloys the R.S.M.E. were produced by the severe deformation. That is, the R.S.M.E. is provided to such a shape memory alloy by severely deforming the alloy under its martensite condition to introduce an irreversible defect such as a dislocation into the alloy which does not disappear by the reverse transformation.
It is also disclosed in British Pat. No. 1,315,652 to provide the R.S.M.E. to Ti-Ni alloy by severely deforming the alloy at a temperature above the martensitic transformation temperature. The deformation is regenerated by cooling the alloy below the martensitic transformation temperature. Takezawa and Sato also disclose that the R.S.M.E. can also be generated by heating deformed specimens of Cu-Zn-Al alloy under constraint. The similar manner is disclosed in the paper "Effect of Applied Stress on the Character of Reversible Shape Memory in Cu-Zn-Al Alloy" by Takezawa, Edo and Sato, in the same publication p.p. 661-666.
According to the heating method under constraint by Takezawa et al., the heating temperature is comparatively low, for example, 100.degree. C. as disclosed in the paper, page 662, line 7.
In another paper, J. Japan Inst. Met., 43 (1979), p.229, which is cited in the paper of Takezawa et al. it is disclosed that the heating temperature is about 60.degree.-180.degree. C.
However, the low heating method by Takezawa cannot provide to the Ti-Ni alloy the R.S.M.E. sufficient in the amount of shape memory.